Equine infectious anemia (EIA) is distinctive among lentiviral diseases in that infected horses often develop lifelong subclinical infectious following initial disease episodes. Thus, EIA provides opportunity to study host-virus interactions in a persistent lentiviral infection in which disease is successfully controlled by the host. The reservoirs of EIAV during subclinical infection are not known. Viral DNA, detected by the in situ polymerase chain reaction (PCR), will be used as a marker for infected cells during asymptomatic infection, and these cells will be identified using immunohistochemistry (IHC) for cell-specific antigens. Latency in these cells will be determined using IHC to demonstrate the absence of viral capsid protein. The absence of mRNA in infected cells will constitute evidence of gene restriction at the level of transcription. This data will help characterize the relationship between tropism, viral replication, and the expression of clinical disease, and provide the basis for more specific experiments that examine control mechanisms in EIA.